What is the minimum slope for a metal roof?

There are three, and the code gives all of them in one sentence. R905.10.2: lapped non-soldered seams without sealant need 3:12, lapped with applied lap sealant need 1/2:12, and standing seam systems need 1/4:12. Same metal, twelve to one, decided by the seam.

Can I put a metal roof on a low slope?

Standing seam, yes, down to 1/4:12. A lapped panel without sealant, no, not below 3:12. This is the commonest place a metal roof gets specified wrong, because the material is chosen first and the seam is treated as a finish rather than as the thing that sets the permitted pitch.

How do I calculate metal roof panels?

On covering width, not panel width. Divide the eave length including both rake overhangs by the covering width and round up, then double it for two roof planes. A 38 in panel covering 36 in on a 42 ft eave is 14 panels a side.

Why is the panel wider than what it covers?

Because the difference is the seam. On a 38 in panel covering 36 that is 5.3% of every sheet, and it is not waste: nothing is cut off, the metal simply becomes joint. It also never appears as a line on a quote, so it is worth checking which width a price is quoted against.

Does panel length matter?

More than most people expect on a long slope. A panel that reaches eave to ridge in one piece has no end lap, and the end lap is where metal roofs leak, because it runs across the flow of water rather than with it. On a 15.8 ft run a 20 ft panel removes every horizontal joint on the roof.

Can metal go over spaced sheathing?

R905.10.1 permits solid or spaced sheathing, which is unusual: most coverings need a solid deck. Whether your specific panel can span between purlins is the manufacturer's call and depends on the profile, and it changes the deck cost as much as the panel choice does.

Is metal roofing worth it over asphalt?

This page will not answer that, because it turns on a service life comparison and a price comparison and neither can be sourced honestly here. What it will tell you is what each is permitted on: asphalt needs 2:12 and doubles its underlayment below 4:12, while standing seam metal goes to 1/4:12. On a low-pitch roof that is not a preference, it is the whole decision.

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Metal Roof Calculator

R905.10.2, three minimum slopes for the same metal

Metal panel appears three times in the IRC's slope rules, at 3:12, 1/2:12 and 1/4:12. Same material, twelve to one, and the only difference is how the seam is made.

Metal roof takeoff

Building length

Building width

Pitch, rise per 12

Eave overhang

Seam type (this sets the minimum slope)

Panel width (the sheet)

Covering width (what it covers, off the profile)

Panel length available (0 to cut to the run)

Ridge length (0 to skip trim)

Waste allowance (ours, plain planes)

Slope minimums are quoted from IRC R905.10.2. Panel dimensions are yours, off the profile, because metal panel geometry is proprietary and varies by manufacturer.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Every minimum in R905, on one axis

Worked example

A 40 ft by 28 ft roof at 4:12 with a 12 in overhang, standing seam, 38 in panels covering 36.

Standing seam is permitted down to 1/4:12, so 4:12 clears it easily. So does every other seam, which is worth knowing because at this pitch the seam is a choice rather than a constraint.
Now drop the same roof to 2:12 and pick a lapped panel with no sealant. R905.10.2 does not permit it: that seam needs 3:12. The same metal in a standing seam is fine at 2:12, and so is lapped with sealant. Twelve to one, on the joint alone.
The panel is 38 in wide and covers 36. That 5.3% is not waste and will not come off with a better cut. It is the seam, and you pay for it on every panel.
Fourteen panels a side, 28 in total, at 36 in covering across 40 ft plus the overhang.
And the run is 15.8 ft eave to ridge, so a 20 ft panel reaches in one piece. That removes every end lap, which is the joint most likely to leak because it runs across the flow rather than with it.

The seam decides three things at once: what pitch you are allowed, how much of each panel you pay for and never see, and where the roof is most likely to leak. It is not a finish.

The formula

Panels are counted on covering width, which is not the width you are buying:

panels per side = ⌈(L + 2 × overhang) × 12 / covering width⌉

covering width: what one panel adds to the roof, off the profile drawing
panel width: what you pay for. The difference is the seam
L + 2 × overhang: the eave length including both rake overhangs

The gap between the two widths runs from about 5% on a wide standing seam profile to considerably more on a narrow one, and it never appears as a line on a quote. It is not waste, because nothing is cut off; it is simply the part of each sheet that becomes joint.

Panel dimensions are yours because there is no standard

Metal roof panel profiles are proprietary. Rib spacing, rib height, panel width and covering width all vary by manufacturer, and there is no free public standard that fixes them the way ASTM roughly fixes brick. So this page asks for both widths off the profile rather than assuming a module.

What the code does fix is the slope, and it fixes it three ways. R905.10.2 is one sentence with three numbered items, and it is quoted in full on this page. Lapped without sealant needs 3:12, lapped with sealant needs 1/2:12, and standing seam needs 1/4:12.

That is a factor of twelve inside one material. It is the widest internal spread of any covering in Chapter 9, and it exists because the three seams shed water in genuinely different ways: a lap relies on gravity beating capillary action, a sealed lap adds a barrier, and a standing seam lifts the joint above the water plane entirely.

R905.10.1 also permits metal panels on spaced sheathing, which almost nothing else on this site allows. Whether your panel can span between purlins is the manufacturer's call and it changes the deck cost as much as the panel does.

Read from Seattle's adopted code, which prints IRC Chapter 9 in full, because codes.iccsafe.org returns HTTP 403 to any automated request.

Prices for roof coverings and tear-off

The published figures for roof coverings and tear-off are on the roof replacement cost calculator, with the byline, the publication date and the caveats that go with them. They are not repeated here, because the same table on two pages is worth less than one table you can find.

This site publishes no price of its own. Why, and what that means for these figures.

The end lap is where a metal roof leaks

Metal roofs have very few joints, which is most of the point of them. The joints they do have are not equal.

A side lap or a standing seam runs with the water, down the slope, from ridge to eave. Water arriving at it is already travelling parallel to it and has no reason to cross it.

An end lap runs across the water, horizontally, where one panel stops and the next begins up the slope. Every drop on the roof above that line has to get past it, and on a low pitch it arrives slowly and with time to find a way in.

Which is why panel length is worth more than panel price on a long slope. A panel that reaches eave to ridge in one piece has no end lap at all. On the worked example the run is 15.8 ft, so a 20 ft panel eliminates them; a 16 ft panel would too, and a 12 ft panel would put a horizontal joint across the whole roof.

The calculator says how many rows your panel length produces, and says when the answer is one.

What metal does to everything under it

Panel count is the easy half. A metal roof changes several things below it that a shingle roof does not.

The deck. R905.10.1 permits solid or spaced sheathing. Spaced supports mean purlins rather than plywood, which is a different deck cost in both directions: cheaper in material, more particular about spacing, and entirely dependent on the panel's own span rating.

The movement. Metal expands and contracts far more than the structure under it, along the length of every panel, every day. Standing seam systems use concealed clips that let the panel slide; exposed-fastener panels do not, which is why their fastener holes elongate over years and why they are the ones that need re-screwing.

The condensation. A cold metal underside in a humid attic makes water without any rain being involved. That is a ventilation and vapour question rather than a roofing one, and the ventilation calculator covers the code side of it.

And the ice barrier, where it is required. R905.1.2 measures it from a point 24 inches inside the exterior wall line, which is a horizontal measurement converted to slope length, and the roofing calculator does that arithmetic.

Frequently asked questions

What is the minimum slope for a metal roof?: There are three, and the code gives all of them in one sentence. R905.10.2: lapped non-soldered seams without sealant need 3:12, lapped with applied lap sealant need 1/2:12, and standing seam systems need 1/4:12. Same metal, twelve to one, decided by the seam.
Can I put a metal roof on a low slope?: Standing seam, yes, down to 1/4:12. A lapped panel without sealant, no, not below 3:12. This is the commonest place a metal roof gets specified wrong, because the material is chosen first and the seam is treated as a finish rather than as the thing that sets the permitted pitch.
How do I calculate metal roof panels?: On covering width, not panel width. Divide the eave length including both rake overhangs by the covering width and round up, then double it for two roof planes. A 38 in panel covering 36 in on a 42 ft eave is 14 panels a side.
Why is the panel wider than what it covers?: Because the difference is the seam. On a 38 in panel covering 36 that is 5.3% of every sheet, and it is not waste: nothing is cut off, the metal simply becomes joint. It also never appears as a line on a quote, so it is worth checking which width a price is quoted against.
Does panel length matter?: More than most people expect on a long slope. A panel that reaches eave to ridge in one piece has no end lap, and the end lap is where metal roofs leak, because it runs across the flow of water rather than with it. On a 15.8 ft run a 20 ft panel removes every horizontal joint on the roof.
Can metal go over spaced sheathing?: R905.10.1 permits solid or spaced sheathing, which is unusual: most coverings need a solid deck. Whether your specific panel can span between purlins is the manufacturer's call and depends on the profile, and it changes the deck cost as much as the panel choice does.
Is metal roofing worth it over asphalt?: This page will not answer that, because it turns on a service life comparison and a price comparison and neither can be sourced honestly here. What it will tell you is what each is permitted on: asphalt needs 2:12 and doubles its underlayment below 4:12, while standing seam metal goes to 1/4:12. On a low-pitch roof that is not a preference, it is the whole decision.

Check these numbers yourself

2018 Seattle Residential Code, Chapter 9 Roof AssembliesSeattle Department of Construction and Inspections. Read 2026-09-08. Source of R905.10.1 and R905.10.2 and of every other minimum deck slope in the table on this page, which gathers eighteen figures the chapter states one covering at a time.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Panel dimensions are yours, off the profile. Metal roof panel geometry is proprietary and no free public standard fixes panel width, covering width or rib spacing, so this page multiplies your numbers rather than assuming a module. Whether a panel may span spaced supports is the manufacturer's call. The slope minimums quoted are from one city's adopted IRC and your jurisdiction may have amended them.